Digital reproduction of optical film soundtracks

ABSTRACT

An apparatus for playback of an analog optical sound track comprises a transport means for transporting a film including an analog optical sound track. A scanning means generates an image signal of only the analog optical sound track. An alignment means aligns the scanning means such that the image signal of the analog optical sound track substantially fills a width of the scanning means. A processor processes the image signal to form an audio output signal.

[0001] This invention relates to the reproduction of optically recordedanalog sound tracks and in particular to the restoration of recordedsignal quality.

BACKGROUND

[0002] Optical recording is most common format employed for analogmotion picture sound tracks. This analog format uses a variable areamethod where illumination from a calibrated light source is passedthrough a shutter modulated with the audio signal. The shutter opens inproportion to the intensity or level of the audio signal and results inthe illumination beam from the light source being modulated in width.This varying width illumination is directed to expose a monochromaticphotographic film which when processed, for example, results in a blackaudio waveform envelope surrounded at the waveform extremities by asubstantially clear or colored film base material. In this way theinstantaneous audio signal amplitude is represented by the width of theexposed and developed film track. FIG. 1 depicts in greatly simplifiedform an arrangement for recording a variable width analog audio soundtrack.

[0003] A second method can be employed for analog motion picturesoundtracks where the audio signal causes the total width of thephotographic audio track to be variably exposed. In this method, termedvariable density, the exposure of the complete track width is varied inaccordance with the intensity of the audio signal to produce a trackwhich varies transmission, for example, between substantially clear orcolored base film material and low transmission or high density areas ofexposed and developed photographic material. Thus the instantaneousaudio signal amplitude is represented by a variation in the transmissionof illumination though the exposed and developed film track width.

[0004] Hence with either variable density or variable area recordingmethods the audio modulation (sound) can be recovered by suitablygathering, for example by means of a photo detector, illuminationtransmitted through the sound track area.

[0005] These analog film sound recording techniques can be subject toimperfections, physical damage and contamination during recording,printing and subsequent handling. Since these recording techniques usephotographic film, the amount of light used in recording (Density) andthe exposure time (Exposure) are critical parameters. The correctdensity for recording can be determined by a series of tests todetermine the highest possible contrast whilst maintaining a minimizedimage spread distortion.

[0006] Image spread distortion results when a spurious fringing image isproduced beyond the outline of the wanted image. Typically image spreaddistortion results from diffusion of light within the film base, betweenthe halide grains and the surrounding gelatin. This scattering of lightcauses an image to be formed just beyond the exposed area. Optimalnegative and positive density and exposure can yield a clean sharp welldefined image. However, with variable area recorded negatives, imagespreading causes the peaks of the audio modulation envelope appear to berounded while the valleys of the envelope appear to be sharpened anddecreased in width. This image distortion causes a non-symmetricalenvelope distortion which translates into both odd harmonic distortionand cross modulation distortion in the recovered audio. As the recordingdensity is increased the image spreading increases and consequentlybecomes evident as sibilance, initially in the higher frequency content,because of the shorter recorded wavelengths. Increasing the recordingdensity further, causes the distortion to become noticeable atprogressively lower frequencies in the recorded spectrum.

[0007] Sound recording film is generally only sensitive blueillumination and employs a gray anti-halation dye to substantiallyreduce or eliminate halation effects. Halation can result fromreflections from the back of the film base causing a secondary, unwantedexposure of the emulsion. Typically a fine grain and high contrastemulsion is used with a control gamma between 3.0 and 3.2.

[0008] The frequency response of these recording methods is determinedby various parameters, for example, the speed at which the shutters openand close, the exposure of the film, and the modulation transferfunction MTF of the film which is directly related to Light diffusion.The higher the exposure time the lower the frequency bandwidth of therecording.

[0009] With these optical recording methods the resulting audio signalto noise ratio can be optimized by use of a high contrast image. Forexample, the darker audio envelope waveshape and the clearer thesurroundings, the cleaner or quieter wilt be the sound. However, thereis a limitation to the possible density at which the film can be exposedat without introducing audio distortion due to image spreading in thefilm emulsion.

[0010] Optimum density presents a compromise between signal to noiseratio and image spread distortion. An optimum density can be determinedby test exposures to find an acceptably low value for cross modulationdistortion resulting from image spreading. Frequently older or archivalaudio tracks are improperly recorded and can exhibit severe distortion.However, often some image spread distortion is tolerated in order toobtain an improved audio signal to noise ratio. FIG. 2 shows a somewhatcomplementary variation of cross modulation distortion with density whenprinting from negative to positive film sound stock.

[0011] In addition to density and image spread distortion otherimperfections can result, for example the density of the exposed orunexposed areas can vary randomly or in sections across or along thesound track area. During audio track playback such density variationscan directly translate into spurious noise components interspersed withthe wanted audio signal.

[0012] A further source of audio track degradation relates to mechanicalimperfections variously imparted to the film and or it's reproduction.One such deficiency causes the film, or tracks thereon, to weave or movelaterally with respect to a fixed transducer. Film weave can result invarious forms of imperfection such as amplitude and phase modulation ofthe reproduced audio signal.

[0013] Analog optical recording methods are inherently susceptible tophysical damage and contamination during handling. For example, dirt ordust can introduce transient, random noise events. Similarly scratchesin either the exposed or unexposed areas can alter the opticaltransmission properties of the sound track and cause sever transientnoise spikes. Furthermore other physical or mechanical consequences,such as the film perforation, improper film path lacing or related filmdamage can introduce unwanted cyclical repetitive effects into thesoundtrack. These cyclical variations can introduce spuriousillumination and give rise to a low frequency buzz, for example havingan approximately 96 Hz rectangular pulse waveform, rich in harmonics andinterspersed with the wanted audio signal. Similarly picture area lightleakage into the sound track area can also cause image related audiodegradation.

[0014] A German application DE 197 29 201 A1 discloses a telecine whichscans optically recorded sound tracks. The disclosed apparatus scans thesound information signal and applies two dimensional filtering to theoutput values. A further German application DE 197 33 528 A1 describes asystem for stereo sound signals. An evaluation circuit is utilized toprovide only the left or the right sound signal or the sum signal ofboth as a monophonic output signal.

[0015] Clearly an arrangement is needed that allows optically recordedanalog audio sound tracks to reproduced and processed to not onlysubstantially eliminate the noted deficiencies but to enhance thequality of the reproduced audio signal.

SUMMARY OF THE INVENTION

[0016] In an inventive arrangement an apparatus for the playback of ananalog optical sound track comprises a transport means for transportinga film including an analog optical sound track. A scanning meansgenerates an image signal of only the analog optical sound track. Analignment means aligns the scanning means such that the image signal ofthe analog optical sound track substantially fills a width of thescanning means. A processor processes the image signal to form an audiooutput signal.

[0017] In a further inventive method positional variation of an analogoptical sound track on a film is eliminated. The method comprises thesteps of transporting the film including a sound track with an audiorepresentative envelope subject to positional variation. Forming adigital image of the sound track with said audio representative envelopeand aligning the digital image of said sound track with an audiorepresentative envelope and ensuring the positional variation of saidsound track on the film and peaks of the audio representative enveloperemain within the digital image. Processing the digital image toseparate only the audio representative envelope and form therefrom anaudio output signal.

[0018] A further inventive apparatus facilitates azimuth alignment of ascanning means during optical sound track playback. The apparatuscomprises film transport for transporting a film including an analogoptical sound track. A scanning means generates an image signal of onlythe analog optical sound track and is aligned such that an image signalof the analog optical sound track substantially fills a width of thescanning means. An azimuth aligning means positions the scanning meanssuch that opposite peaks of the image of said analog optical sound trackare displayed concurrently with substantially the same magnitude.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a diagrammatic representation of an audio soundtrackusing a variable area recording method.

[0020]FIG. 2 shows relationships between cross-modulation distortion andrecording density.

[0021]FIG. 3 is a block diagram of an inventive arrangement forprocessing optically recorded analog audio sound tracks.

[0022]FIGS. 4A and 4B show a 16 mm film gauge implementation of theinventive arrangement of FIG. 3.

[0023]FIG. 5 shows a scanned gray scale analog image of a variable areaaudio soundtrack subject to certain deficiencies.

[0024]FIG. 6 illustrates a control panel used in accordance with theinventive arrangement of FIG. 3.

[0025]FIG. 7 shows a processed scanned image of an audio soundtrack inaccordance with a further inventive arrangement.

[0026]FIG. 8A illustrates diagrammatic representations of an exemplaryelliptical area of the track image shown in FIG. 7.

[0027]FIG. 8B illustrates the result of a erosion filter processing inaccordance with a further inventive arrangement.

[0028]FIGS. 9A and 9B are charts representing sequences associated withvarious inventive arrangements.

[0029]FIGS. 10A and 10B are diagrams representing a sound track envelopereproduced with an azimuth error in FIG. 10A and corrected in FIG. 10A.

DETAILED DESCRIPTION

[0030] The block diagram of FIG. 3 shows an inventive arrangement forreproducing and processing an optically recorded analog audio soundtrack. Typically a light source 10 is projected onto film 20 whichincludes an audio sound track 25, depicted in FIG. 3 with an exaggeratedwidth dimension. The audio signal my be represented as suggested intrack 25 by means of a variable area recording method, however the audiosignal may also be represented by corresponding variations in densitysubstantially across the width of the sound track area. In aconventional film sound reproducer light from source 10 is transmittedwith varying intensity through film 20 and track 25 in accordance withthe method employed for exposing the sound track. However, the resultingvarying intensity transmitted light is gathered by a photo sensor suchas a photo cell or solid state photo detector. The photo sensor usuallygenerates a current or voltage in accordance with the intensity of thetransmitted light. An analog audio output signal results from the photosensor and this is generally amplified and often processed to alter thefrequency content to improve or mitigate deficiencies in the acousticproperties of the recorded track. However, such frequency responsemanipulation, is generally incapable of remedying the deficiencieswithout adversely effecting the wanted audio content.

[0031] In the inventive arrangement shown in FIG. 3, light from source10 is guided by a fiber optic means (not illustrated) to from aprojected beam of light for illuminating sound track 25. The light ismodulated in intensity by the sound track and is collected by opticalgroup 75. Optical group 75 includes a lens assembly, extension tube andbellows which are arranged to form an image of the complete sound trackwidth across the width of a CCD line array sensor 110 which forms partof camera 100. Camera 100, for example a Basler type L160, is controlledby frame grabber 200, for example, Matrox Meteor II LVDS digital boardwhich synchronizes the image capture and outputting of an 8 bit digitalsignal representing the line scanned image of sound track 25 as the filmmoves continuously through the projected beam of light. The CCD linearray sensor 110 has 2048 pixels and provides a parallel digital outputsignal 120, quantized to 8 bits and capable of operating with a bit ratein the order of 60 MHz.

[0032] The digital image signal 120 represents successive measurementsacross the width of the sound track which are captured as an 8 bit grayscale signal representing the instantaneous widths of exposed andunexposed areas of the sound track. This continues succession of trackwidth images or measurements are stored by an exemplary RAID system 300as a continuous digital image of the optical track.

[0033] An operating system can be resident in controller 400 or asdepicted by block 405 which provides the user with a visual menu andcontrol panel presentation on display 500. Controller 400 can a personalcomputer or can be implemented as a custom processor integrated circuit.However, the computer controller must support the high transfer ratesassociated with the camera data and requires at least 512 MB of RAMtogether with an Ultra SCSI 160 interface that can sustain the hightransfer rates. In addition a dual processor computer can allow parallelprocessing which can increase both processing speed and performance.

[0034] Camera 100 has a line array CCD sensor with 2048 pixels andprovides an 8 bit parallel digital output signal, 120, in accordancewith LVDS or RS 622 output signal formats. The use of a 2048 pixel linearray sensor provides sufficient resolution to capture the soundtrackenvelope image without significant frequency response distortion. Inaddition the camera can be controlled by a frame grabber 200, which inaddition provides synchronization to NTSC or HD television sync pulsesvia sync interface 250, and also permits an output data rate sufficientto capture sound track images at normal operating speeds of nominally 24fps.

[0035] Thus under control of frame grabber 200 and responsive to usercontrol from display and keyboard 600 the digital image is transmittedvia a frame capture card 200 for storage on a hard disk memory array300. For example the scanning rates employed in this advantageousarrangement result in an exemplary file size in the order of 4 gigabytes per minute and this bitstream is supplied for storage by a stripedraid system 300 which facilitates storage of the large sound track imagevideo file while providing rapid transfer rates.

[0036] The optical system, bellows extension tube and lens 75 areaccurately positioned to image the standardized recorded trackpositions, however manual adjustments are provided to permit bothfocusing, exposure and image size adjustment or zoom control to allowthe recorded film area to substantially fill the maximum sensor widthwith peak audio modulation. The camera mounting system also facilitatesboth lateral and azimuth adjustments. Lateral adjustment L allowslaterally mis-positioned tracks to be imaged, for example to eliminatesprocket or perforation generated buzz or picture related light spill.Furthermore in severe situations where lateral image adjustment fails toeliminate audible sprocket hole or perforation noise, or picture spill,the camera and lens can be adjusted to substantially fill the sensorwidth with a part of the recorded envelope positioned to avoid theoffending illuminating noise source.

[0037] The selection of tens and optical system requirements aredetermined largely by the 35 mm audio optical track width and the widthof the imager array. A 35 mm optical track has a standardized width of2.13 mm, and the approximate length of the imager is about 20.48 mmbased on a pixel size of 10 microns. Thus to enable the maximum width ofa 35 mm sound track to fill the imager width requires an imagemagnification of about 10:1. Similarly for a 16 mm track having a widthof 1.83 mm, in order to fill the sensor width requires the addition of a56 mm extension tube or bellows.

[0038] In addition to the imaging considerations, the desired bandwidthof the processed audio signal must be considered. For example, if areproduced audio bandwidth of 15 kHz is required, a sampling or imagescanning rate of 30 kHz is needed. Thus with an exemplary sampling rateof 30 kHz, the camera will output 2048 bytes or 8 bit words for eachimage scan (audio track line scan) producing an output data rate of2048*30*1 or 61.4 mega bytes per second. Hence one minute of sound trackrequires approximately 3.68 giga bytes of storage. Such storage capacityrequirements can be provided by an exemplary striped raid system such asan Ultra Wide SCSI 160 drive.

[0039]FIG. 4A illustrates an exemplary magnetic film transportmanufactured by Magna-tech Electronic Co. Inc. which forms the basis forthe inventive scanning arrangement and provides a servo controlled filmtransport system with adequate room for mounting the Line array CCDcamera. A major requirement is that of good film guidance and theprovision of a steady film path to prevent variation of film focus as ittravels between the light source and camera. Through experimentation itwas discovered that optimum film stability for scanning was achieved ata location where the film wraps around a flywheel. Although film imagesurface is curved at the flywheel the use of line array scanner lookingorthogonally and without azimuth errors at the film obviates problems ofdepth of field and sound track inter-modulation, and phasing or flangingdistortions.

[0040] An exemplary flywheel is depicted with a 16 mm gauge film in FIG.4B together with a cranked fiber optic tight guide which facilitatesorthogonal illumination of the film without obscuration by the cut awayflywheel center. In an alternative arrangement, illustrated in FIG. 4C,an exemplary flywheel provides support for a major part of the filmwidth and obviates the requirement for the cranked light guide shown inFIG. 4B. In this arrangement the 16 mm gauge film is supported by theflywheel over the majority of the film width with the exception of anominally 3 millimeter edge region which contains the sound track ortracks. Similarly when operating with 35 mm gauge film an edge region ofabout 8 mm containing the sound modulation extends beyond the exemplaryflywheel of FIG. 4C. The wrapping action of the film around the flywheelforms a partially cylindrical structure (CS) which provides rigidity andsignificant stiffness and hence resistance to edge deviation or fluttereffects. In this way the advantageous wrapped positioning of the soundtrack area relative to the flywheel ensures a stable film edge anddefocusing of the image is largely precluded.

[0041] The inventive film sound processing system is activated bykeyboard 600 or mouse selection of an icon (Digital AIR) which resultsin a Windows® like control screen arrangement presented on displayscreen 500. Various operating modes such as Preview, Record, Stop,Process and Export are presented as tool bar functions in a border areaof the display. Initially the Preview mode can be selected from the toolbar functions which advantageously starts the sound track in motion andforms a sound track image on display screen 500. The gray scale imageallows alignment of camera and optics to the recorded sound track.Optical group 75 is adjusted to ensure that peaks of the sound trackimage substantially fill the imager 110 width and to provide good imagesignal to noise ratio by ensuring proper CCD exposure which can differbetween negative and positive prints and is also dependent on the typeof film stock.

[0042] Advantageously the real time mage provides not only pictures ofthe sound track but also shows the presence of interference generatingillumination emanating from the sprocket holes, or the picture areawhich can contaminate the sound track. This unwanted light ingress canbe eliminated by using the on screen camera image to permit manipulationof optical group 75 to remove such unwanted audio contributions bycarefully framing the soundtrack using picture zoom, pan and tilt. Inaddition the sound track image can be examined in detail byelectronically magnifying selectable sections of the display envelope topermit camera azimuth alignment when reproducing a test film known as abuzz track. The magnified image is presented with an electronicallycursor line which permits the evaluation of any time or phase differencebetween peaks in the modulation envelope. With optimized azimuthalignment modulation peaks appear concurrently with substantially equalmagnitude but opposite polarity. An optimum azimuth adjustment willproduce concurrently maximized envelope peaks. Misalignment of azimuthbetween the camera an the sound track can result in an image whichcaptures temporally different audio information, such as can occur witha stereo audio track pair. FIG. 10A is diagram representing a soundtrack envelope reproduced with an exemplary and exaggerated azimutherror. Shown on the same time axis of FIG. 10A is a processed orelectronically cored image showing the temporal displacement resultingfrom an azimuth error between the camera imager camera and the soundtrack. FIG. 10B is the same envelope image as FIG. 10A but reproducedwithout an azimuth error, and shown below on the same time axis is theelectronically cored image which indicates that the envelope peaks havebeen scanned substantially concurrently and are of similar amplitudes.

[0043] An example of a Preview mode sound track image is shown in FIG.5. The gray scale picture in FIG. 5 is of a duplicate negative soundtrack which includes various impairments. For example, on the right sideof the sound track image unwanted illumination can be seen emanatingfrom film perforations, a defect indicative of misalignment duringduplication. In addition the sound track has a reduced width and showslateral scratches probably incurred on the original negative. Hence theadvantageous real time sound track image permits rapid visual alignmentof the camera and optics, rather that reliance on acousticallydetermined positioning. The scanning alignment sequence is depicted inthe sequence chart of FIG. 9A. The sound track image facilitates thesubstantial elimination of deficiencies resulting from priormisalignment. Following camera image optimization, framing, focus,exposure, etc., the Record mode is selected from the tool bar and thesound track is scanned, digitized as exemplary 8 bit words and stored inmemory 300. Upon completing the scanning and storage steps the digitalsound track image is processed by selecting the Processing mode from thetool bar.

[0044] The processing control panel shown in FIG. 6 allows the operatorto select and optimize film specific processing to be performed on thestored sound track image thereby obviating the potential for damagingthe film material during repeated play back for optimization.Advantageous processing algorithms resident, for example in controller400 or as depicted within block 410 are selected from the on screen menuvia keyboard 600 and applied to data selectively retrieved from thestored digital image in system 300. The algorithms employed to remedycertain sound track deficiencies will be explained, however, thecorrective processing sequence is depicted in the chart of FIG. 9B. Theprocessed and renovated digital signal is converted for outputting asdigital audio signal 450 with selectable exemplary formats such as WAV,MOD, DAT, DA-88.

[0045] Having stored the complete soundtrack as a digital image theinventive Processing mode is selected from the on screen tool bar. Theprocessing control panel shown in FIG. 6 allows the operator to selectand optimize processing specific to the stored sound track image. Forexample film gauge is selectable, together with the film type, positiveor negative and audio modulation method for example, unilateral variablearea, bilateral variable area, dual bilateral variable area, stereovariable area or variable density. The advantageous processingalgorithms are selected from the on screen menu and applied to thestored digital image accessed from storage system 300 for processing bythe CPU or a DSP card of controller 400.

[0046] Sound track deficiencies can result from the various causesdescribed previously. However, more specifically, dirt, debris,transverse or diagonal scratches or longitudinal cinches in a negativecan produce white spots when printed. These flaws generate clicks andcrackles. Such white spots tend to affect the dark areas of the trackand are more noticeable during quiet passages whereas noise occurringduring loud passages often originates in the clear areas of the print.Low frequency thuds or pops often result from relatively large holes orspots in a positive soundtrack formed as a consequence processingproblems. Hiss can result from a grainy or slightly fogged track area.Sibilance yields spitting S sounds and is particularly objectionable.Typically sibilance results from image spreading within the photographicemulsion of variable area recordings and gives rise to cross modulationdistortion of audio signals recorded on the track.

[0047] Although the scanned audio track is represented as a continuousenvelope image it was advantageously recognized that sections of theenvelope image can be read from memory 300 and configured in RAM forprocessing using spatial image techniques. An first algorithm wasdeveloped using Matlab® to facilitate loading the audio envelope imageas matrix of values to permit the use of spatial image processing. Bygathering small consecutive pieces of the audio envelope to form spatialimage sections it is possible with a second algorithm to identify andeliminate extraneous pixels that differ from surrounding pixels. Withoutprocessing, such extraneous pixels can produce transient noise in thereproduced audio signal. In this second algorithm a small mask or windowcomprising, for example, 3×3 pixels is formed with groups of threepixels values from three adjacent line scans. This window is moved orstepped across the spatially configured sound track image data with thepixel of interest, or subject pixel centered in the window. If the valueof the subject pixel differs from the value of the surrounding pixels itis replaced with the value of the surrounding pixels. Thus thisalgorithm is suited to use with signals that have been subject digitalthreshold processing, which will be described, where isolated, contrarydata values can in general be associated with erroneous and ultimatelyaudio noise generating consequences. Hence such contrary data values arereplaced by the predominate value within the window. Thus each pixel ofthe scanned audio track is tested and replaced to form a processedsoundtrack image in RAM. In edge areas padding is applied to preventerroneous pixel replacement.

[0048] Scratches across sound track can produce transient or impulsivenoise effects such as loud pops or clicks. The simple rule of pixelreplacement described in the second algorithm is less effective withcontiguous contrary value pixels. However, this form of transient noiseis advantageously eliminated by a third algorithm which is applied tospatially configured track image sections of the stored exemplary 8 bitdigital envelope signal. This third algorithm uses a further spatialimage processing technique to derive median values for each pixel ofeach image section across the width of the track. These median valuesare then used to replace the scanned image data across the track area.The median filter is implemented by an exemplary mask or windowcomprising, for example 9×9 pixels, which is progressively stepped,pixel by pixel across a spatial representation of the audio envelopedata. The center of the window represents the pixel to be corrected. Thepixel values of the track image positioned under the window are sortedor ranked in amplitude order. The middle value of the rank ordered setis then substituted for the actual track image value of the center pixelof interest, this process is then repeated for the next pixel across thewidth of the spatially configured track image. Ultimately every pixelrepresenting the scanned audio track is evaluated and if necessaryreplaced forming a processed soundtrack image in RAM.

[0049] Other mask or window sizes and shapes can be advantageouslyemployed to favor formation of median values. For example a 3×6 maskformed from three successive image scans across the sound track widthwill form a pixel neighborhood that favors the track width in theformation of the median value. Alternatively the mask or window can beadvantageously favor formation of a median value from a pixelneighborhood extending over a greater number of successive scans butoccupying less track width for example by use of a 9×3 mask. In additionexemplary masks can be constructed to provide diagonal weighted imageprocessing.

[0050] Because the median filter window analyzes data from pixel groups,with some occurring in adjacent line scans, an amount of blurring ordata smoothing can result because the middle value of the rank orderedset can be representative of a data value occurring at a differentspatial and or temporal scanned location. However, this smoothing effectcan be compensated with a two dimensional high pass filter which cansharpen or substantially restore the image. The median filter process iscomputationally intense and therefore time consuming but can beoptimized by recognizing that certain values within the window will notchange from step to step.

[0051] Following median filtering of the audio envelope image data whichremoves aberrant values a further operation is performed termedContrast. The Contrast process advantageously recognizes that thevariable area recording method employs only two states, one to representthe audio envelope, the second to represent the envelope's absence. Thusthe sound track has some areas that are substantially clear and othersthat are opaque. Advantageously processing screen FIG. 6 allows sectionsof the stored image to be previewed, by selecting button A, and viewingthe resulting image as contrast slider B is varied. Contrast slider Ballows a threshold value of a further software algorithm, or hardwareimplementation to be varied about a nominal center range decimal valueof 127 for an exemplary 8 bit range of image values scanned from thesound track. The algorithm classifies the pixels according to theirintensity value and splits the range of values in two. Thus for imagesdigitized with values less than the selectably adjusted threshold theactual scanned digital value, or median filtered value, is replaced witha new low digital value, for example representing decimal 0, andsubstantially equal to black or zero film transmission. Similarly fordigitized images values greater than the adjusted threshold value theactual value is replaced with a new high value substantially equal towhite or decimal value 255. In this way grayscale variations in thenominally clear and opaque film areas are removed and defects causingvariable light transmission through the track are eliminated. Thisdigital thresholding or binarization method re-quantizes the storeddigital audio envelope image into 2 states, represented by one bit.However, although contrast slider B offers the visually apparent abilityto remove or eliminate dirt, scratches and artifacts from the on screenpreview image, the result must be balanced, and acoustically judgedagainst any consequential, unintentional and unwanted changes to theaudio content.

[0052] Vertical slider bar C provides access to 10 sections of therecorded image data, assigned on the basis of file duration, number offrames or running time. These 10 sound track sections allow the effectof differing digital threshold values, determined by contrast slider B,to be evaluated on track areas containing both loud and quiet passages.The advantageous digital thresholding or binarization process improvesthe signal to noise ratio of the image signal and aids in theidentification of the edges of envelope image. FIG. 7 shows a section ofa soundtrack image subject to digital threshold processing.

[0053] Image spread distortion effects variable area recordings andresults in objectionable audio sibilance. Image spread distortionresults during recording from scattering of light causing the growth ofthe image or fringe beyond the actual image outline. Since the spreadingis exposure dependent the effect is initially evident in higherfrequency or shorter wavelength audio content. Image spreading causespeaks of the audio modulation envelope to become rounded while thevalleys of modulation envelope appear to be sharpened. Thus the soundimage envelope becomes non-symmetrical and causes harmonic distortionand cross modulation of the audio content.

[0054] Once again spatial image processing techniques are advantageousused to significantly reduce or substantially eliminate sound trackimpairment due to image spread distortion. Various spatial imageprocessing algorithms can be used to remove the envelope asymmetrycaused by image spreading. In a exemplary algorithm Sobel filters can beused to find the outline of the audio envelope which is then furtherprocessed to identify valleys and peaks. In accordance with the slopeand amplitude of the envelope, a weighted number of pixels are added tothe envelope image and operational control can be provided a graphicuser interface to control the weights of the corrective additions.

[0055] In a fourth advantageous arrangement morphological erosionfiltering is employed to significantly reduce or eliminate the effect ofimage spread distortion of the audio track envelope. Erosion filteringis performed by analyzing each pixel of the spatially configuredenvelope image, usually in binary or thresholded form, with astructuring element, for example a 3×3 array having values of either oneor zero. The structuring element is stepped over each pixel of theenvelope spatial image with the center of the element covering the inputpixel of interest. If the structuring element is an 3×3 array of onesthen the output value of the pixel of interest is determined by thecorrespondence of the envelope pixel neighborhood surrounding the pixelof interest under the array, with the values in the array. If all theneighborhood pixels and the pixel of interest match the exemplary 3×3array of ones, then the output value of the pixel of interest is notchanged. However, as soon as any part of the 3×3 array straddles an edgein the exemplary thresholded envelope image, the pixel of interest ischanged from a one to a zero. Thus with the exemplary 3×3 structuringelement an envelope edge between white and black is detected by aleading one of the neighborhood pixels causing the adjacent centerpixel, or pixel of interest, to assume the same value as the leadingneighborhood pixel, thereby causing the white to black transition tomove, shrink or erode into the white or binary one area.

[0056] With the exemplary 3×3 structuring element edges of the audioenvelope are eroded by one pixel. The amount of image spreading canexceed the width of one pixel, however a second pass of the erosionfilter will remove a second pixel but at the expense of processing time.In a further advantageous arrangement varying amounts of image spreadcorrection can be selected, as indicated in area D of FIG. 6, with thedesired degree of correction performed in a single processing step.Greater amounts of erosion can be achieved by use of a largerstructuring element, for example with a 5×5 array, erosion of two pixelsis achieved corresponding to the selectable correction of a mediumdegree of distortion. Similarly processing with a 7×7 structuringelement erodes three pixels and represents the correction of severdistortion.

[0057] Morphological erosion filtering can be performed with a softwarealgorithm, for example developed using Matlab®, or alternatively thefilter function may be implemented with hard wired logic. Howeverimplemented, the representation of the audio track envelope in thespatial domain permits the advantageous use of erosion filteringtechniques to mitigate image spread distortion, largely eliminate crossmodulation and restore the audio track fidelity.

[0058]FIG. 8A is a diagrammatic representations of exemplary ellipticalarea 8 of the threshold processed track image depicted in FIG. 7 andshows both white squares representing pixels or digital sample valuesand gray squares representing pixels or digital sample values from theblack areas of FIG. 7. FIG. 8A includes a representation of exemplary3×3 structuring element SE which is formed as follows, 0 1(A) 0

0 1(X) 0 0 1(B) 0

[0059] having one values or active cells, A, X and B in the centercolumn, with the pixel of interest marked with an (X). The structuringelement is stepped across the spatial representation of the track image,pixel by pixel as indicated by the arrow. Because this structuringelement has only three active cells, the processed value of center pixelX is determined by the laterally adjacent pixel neighborhood as shown,where the center value X is determined by the following erosionalgorithm, if (X • A • {overscore (B)}) + (X • {overscore (A)}• B) then_X′ = {overscore (X)} else_X′ = X

[0060] With this exemplary structuring element the output value of thepixel of interest is determined by the correspondence of the track imagepixel neighborhood adjacent to the pixel of interest under thestructuring element. If the adjacent neighborhood pixels and the pixelof interest match the structuring element, then the output value of thepixel of interest X′ is not changed. However if either track imagevalues under cells A or B fail to match then the pixel of interest X′ ischanged to the complementary value, for example zero.

[0061] The enlarged processed track image of FIG. 8A shows theadvantageous structuring element SE positioned to perform erosionfiltering with FIG. 8B showing the resulting eroded image where erodedpixels are shown as white blocks with broken outlines with the currentpixel of interest depicted with by a * symbol. The solid white squaresthat represented pixel values in FIG. 8A are omitted from FIG. 8B toallow the eroded pixels greater visibility.

[0062] Following the advantageous use of spatial image processingtechniques the processed envelope image is converted back to soundsignal by a further advantageous algorithm. The conversion algorithmsums the number of black pixels, for a negative track, or white pixelsfor a print, that represent the audio envelope for each line scan. Thisnumber of active pixels, representing the instantaneous amplitude whichis then subtracted from the maximum amplitude value, for example 2048,which represents the total sensor pixel count. The resulting differencerepresents the instantaneous audio amplitude. Clearly the converseprocess is also possible where a nominally smaller number ofnon-envelope representative end pixels are counted and subtracted fromthe total sensor pixel count with the result representing theinstantaneous audio amplitude. This audio amplitude value is then scaledto an appropriate audio signal format range. For example, using a 16 bitWAV file format the renovated audio values are scaled to fit a range of−32767 to +32768, where 0 represents DC. This audio conversion algorithmwas developed using a Matlab® image processing toolbox. The Algorithmalso includes a routine that prepares header appropriate for the fileformat and provides a streaming buffer to receive the WAV data followingconversion. In addition to WAV formatted files a variety of other audiofile formats are available including AIFF, MOD, DAT, DA-88 and DA-98HR.

[0063] In a further inventive aspect film weave which causes the soundtrack to vary in position relative to the audio transducer isadvantageously corrected. The effects of film weave can appear asvarious types of modulation of the audio signal. Often an amplitudemodulation results where the modulation is representative of the rate offilm weave. In severe cases the reproduced audio signal can be subjectto a low pass filtering effect where the cut off frequency is modulatedby the film weave. In accordance with the inventive arrangement thepresence of film weave results in the instantaneous audio envelope imagealso weaving or meandering on the sensor, however, this positional imagevariation only results in a variation of the pixels representing anenvelope image absence. For example, in a negative track these pixelswould represent a clear or high transmission part of the track and arepositioned at the end regions of the array.

[0064] During the initial camera alignment the track image is observedat several film locations and if film weave is apparent the imagecentering can be adjusted to position the nominal center of wanderingsound track path in the middle of the display image. The image size isthen adjusted such that audio envelop peaks occurring at the maximumexcursions of the track wander do not exceed the width of the CCD linearray. Thus having centered the wandering envelope image the numbers ofpixels at each end of the array are substantially similar for thecentered track. Hence it can be appreciated that as the film weaves onlythe numbers, or distribution of the end (non envelope) pixels vary.However, the envelope pixel count, which represents the envelopeamplitude, remains substantially constant because the envelope imagemoved, but remained on the sensor array. Thus the algorithm forconverting the envelope image into an audio value advantageouslyeliminates and corrects the effects of film weave.

What is claimed is 1) an apparatus for analog optical sound trackplayback, comprising means for transporting a film including an analogoptical sound track; scanning means generating an image signal of onlysaid analog optical sound track; means for aligning said scanning meanssuch that said image signal of said analog optical sound tracksubstantially fills a width of said scanning means; and, a processor forprocessing said image signal to form an audio output signal. 2) Theapparatus of claim 1 wherein said scanning means comprises a line arrayCCD camera having an image width defined by a number of pixels. 3) Theapparatus of claim 2, comprising a video display for viewing said imagesignal to permit adjustment of said alignment means such that audiopeaks in said analog optical sound track substantially fill said imagewidth. 4) The apparatus of claim 3, wherein said aligned scanning meansconverts each pixel of said pixels representing of said analog opticalsound track width to a digital value. 5) The apparatus of claim 4,wherein said processing means separates said pixels representing analogoptical sound track width into a first group of pixels having digitalvalues representative of an audio signal present in said analog opticalsound track and a second group of pixels having digital valuesrepresentative of unused sound track area. 6) The apparatus of claim 5,wherein said processing means includes an algorithm for generating fromsaid first group of pixels a signal representative of an instantaneousaudio signal amplitude by subtracting a total number of pixels in saidfirst group of pixels from said number of pixels representing said imagewidth. 7) The apparatus of claim 4, wherein said image signal representssaid analog optical sound track as a digital word. 8) A method forplayback of an analog optical sound track, comprising the steps of: a)scanning said analog optical sound track; b) forming an image signalrepresenting said analog optical sound track during said scanning; c)processing said image signal to form a video display signal; and, d)viewing said video display signal and adjusting said scanning to centersaid image signal representing said analog optical sound track. 9) Themethod of claim 11 wherein said adjusting step comprises, e) filling awidth of said video display signal with a representation of said analogoptical sound track. 10) The method of claim 9 comprising the step of,f) converting said image signal to form an audio signal representativeof said sound track, and, g) listening to said audio signal andadjusting alignment to optimize formation of said image signalrepresenting said analog optical sound track. 11) A method foreliminating positional variation of an analog optical sound track on afilm, comprising the steps of: a) transporting said film including asound track with an audio representative envelope; b) forming a digitalimage of said sound track with said audio representative envelope; c)aligning said digital image of said sound track with an audiorepresentative envelope and ensuring said positional variation of saidsound track on said film and peaks of said audio representative enveloperemain within said digital image. 12) The method of claim 11, whereinsaid forming step comprises imaging said sound track with said audiorepresentative envelope with a line array CCD camera having a number ofpixels defining a width of said digital image. 13) The method of claim12, wherein said aligning step comprises viewing said digital image andadjusting said alignment. 14) The method of claim 12, comprisingseparating [each pixel defining said width of said digital image]according to a digital value of each pixel representing one of only saidsound track and said sound track with said audio representativeenvelope. 15) The method of claim 14, wherein said separating stepcomprises quantizing said digital value of each pixel to have one of twovalues. 16) The method of claim 14, wherein said separating stepcomprises summing said pixels having said digital value representingsaid sound track with said audio representative envelope and subtractingsaid pixel sum from said number of pixels defining said digital imagewidth to form a number representing an instantaneous audio signalamplitude. 17) An apparatus for analog optical sound track playback,comprising means for transporting a film including an analog opticalsound track; scanning means generating an image signal of only saidanalog optical sound track; a first means for aligning said scanningmeans such that said image signal of said analog optical sound tracksubstantially fills a width of said scanning means; a second means foraligning azimuth of said scanning means such that opposite peaks of saidimage of said analog optical sound track concurrently have substantiallythe same magnitude. 18) The apparatus of claim 16, comprising an imageprocessing means for selectively displaying said opposite peaks of saidimage of said analog optical sound track magnified in display sizetogether with an electronically generated cursor line for indicatingcoincidence of said peaks.